Diaminodicyclohexylmethane is an industrially important compound to be used as a raw material for epoxy curing agents and polyurethanes. A diaminodicyclohexylmethane has three isomers, i.e., a cis-cis isomer, a cis-trans isomer and a trans-trans isomer, derived from the cyclohexane ring. It is known that the physical properties of a polymer obtained by using a diaminodicyclohexylmethane vary depending upon the ratio of isomers.
For example, in a polyurethane obtained from diisocyanato dicyclohexylmethane prepared by isocyanating diaminodicyclohexylmethane, it is reported that the mechanical strength becomes excellent as the content of the trans-trans structure increases (Patent Literature 1).
For these reasons, it is extremely important to control the isomer ratio of a diaminodicyclohexylmethane. Various methods have been proposed for controlling the isomer ratio of a diaminodicyclohexylmethane.
As a method for controlling the isomer ratio, for example, a method for controlling the isomer ratio of obtained diaminodicyclohexylmethane by controlling the hydrogenation conditions of a precursor of a diaminodicyclohexylmethane, i.e., a bisaminophenylmethane, is reported (Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5).
As an isomerization method for a diaminodicyclohexylmethane, a method for isomerizing a diaminodicyclohexylmethane by reacting an aldehyde (2 mole equivalents) with a diaminodicyclohexylmethane to obtain a diimine and isomerizing the diimine, followed by hydrolysis, is reported (Patent Literature 1, Patent Literature 6).